https://webmidi-examples.glitch.me/

tinygo-midi.go

package main

import (
	"fmt"
	"machine"
	"machine/usb/midi"
	"time"
)

// Try it easily by opening the following site in Chrome.
// https://www.onlinemusictools.com/kb/

func main() {
	led := machine.LED
	led.Configure(machine.PinConfig{Mode: machine.PinOutput})

	m := midi.New()
	m.SetHandler(func(b []byte) {
		led.Set(!led.Get())
		fmt.Printf("% X\r\n", b)
	})
	chords := []struct {
		name string
		keys []midi.Note
	}{
		{name: "C ", keys: []midi.Note{midi.C4, midi.E4, midi.G4}},
		{name: "G ", keys: []midi.Note{midi.G3, midi.B3, midi.D4}},
		{name: "Am", keys: []midi.Note{midi.A3, midi.C4, midi.E4}},
		{name: "F ", keys: []midi.Note{midi.F3, midi.A3, midi.C4}},
	}
	index := 0
	current := false
	for {
		current = !current
		if current {
			for _, c := range chords[index].keys {
				m.NoteOff(0, 0, c, 0x40)
			}
			index = (index + 1) % len(chords)
		} else {
			for _, c := range chords[index].keys {
				m.NoteOn(0, 0, c, 0x40)
			}
		}
		time.Sleep(500 * time.Millisecond)
	}
}